1. Field of the Invention
The present invention relates to an optical pickup apparatus configured to perform an operation of reading a signal recorded in an optical disc.
2. Description of the Related Art
An optical disc device is widely available, which is capable of performing an operation of reading a signal and an operation of recording a signal with a laser beam emitted from an optical pickup apparatus being applied to a signal recording layer of an optical disc.
As the optical disc device, those using optical discs called CDs and DVDs are widespread in general. As the laser beam to perform the operation of reading a signal recorded in an optical disc of CD standard, infrared light having a wavelength of 780 nm is used, while as the laser beam to perform the operation of reading a signal recorded in an optical disc of DVD standard, red light having a wavelength of 650 nm is used.
A protective layer disposed on an upper face of the signal recording layer in the optical disc of CD standard has a thickness of 1.2 mm, and an objective lens to be used for the operation of reading a signal from the signal recording layer is specified to have a numerical aperture of 0.45. Also, a protective layer disposed on an upper face of the signal recording layer in the optical disc of DVD standard has a thickness of 0.6 mm, and an objective lens to be used for the operation of reading a signal from the signal recording layer is specified to have a numerical aperture of 0.6.
In an optical pickup apparatus, which is configured such that the operation of reading a signal recorded in the optical disc of CD standard and the operation of reading a signal recorded in the optical disc in the DVD standard as described above are performed by a light condensing operation of the same objective lens, a light flux of laser beams incident onto the objective lens in accordance with the wavelength of the laser beam emitted from a laser diode in use is configured to be limited in accordance with the numerical aperture.
In order to perform an operation of changing the numerical aperture, a configuration is such that a wavelength selection filter is used, an aperture stop with different opening diameters is mechanically stopped down or a liquid crystal shutter is used. Also, as a method of performing the operation of reading signals recorded in the optical discs of CD standard and DVD standard using the same objective lens, a method of using an objective lens called bifocal lens having two focuses is also employed.
In the optical pickup apparatus which is configured so as to be able to perform the operation of reading signals recorded in the optical discs of the above-described different standards, two laser diodes which emit laser beams of different wavelengths are included, however, in order to simplify a configuration of an optical system, a laser diode called a two-wavelength laser diode has been commercialized, which is provided, in the same package, with a first laser element configured to emit a first laser beam and a second laser element configured to emit a second laser beam of a wavelength different from that of the first laser beam.
Such an optical pickup apparatus, which is configured so as to perform the operation of reading signals recorded in the optical discs of different standards using the first laser light and the second laser light emitted from the above described two-wavelength laser diode, is described in the Japanese Patent Laid-Open Publication No. 2001-307362, for example.
FIG. 1 is an optical layout diagram of an optical pickup apparatus, which is configured such that the first laser beam and the second laser beam emitted from the two-wavelength laser diode are collected by one objective lens to the signal recording layers provided in the optical discs of the different standards, and a description will be given, as an example, of an optical pickup apparatus configured so as to be able to perform the operation of reading signals recorded in a first optical disc D1 of CD standard and in a second optical disc D2 of DVD standard.
In this description, omitted are optical components such as a quarter-wave plate, which converts the laser light from linearly polarized light into circularly polarized light, and to the contrary, from circularly polarized light to linearly polarized light, an anamorphic lens, which corrects various aberrations included in return light to be applied to a photodetector.
In this figure, reference numeral 1 designates a two-wavelength laser diode, in which a first laser element 2 configured to emit a first laser beam having a wavelength, e.g., 780 nm, which is suitable for reading a signal recoded in the first optical disc D1 of CD standard, and a second laser element 3 configured to emit a second laser beam having a wavelength, e.g., 650 nm, which is suitable for reading a signal recorded in the second optical disc D2 of DVD standard, are arranged on the same semiconductor substrate and provided in the same package.
Reference numeral 4 designates a diffraction grating, on which the first laser beam and the second laser beam emitted from the two-wavelength laser diode 1 are incident and which performs a diffraction action on the second laser beam, and the diffraction grating is configured so as to split the second laser beam to generate a main beam, which is a 0-order beam, and a sub beam, which is a ±first-order beam. The diffraction grating 4 has a diffraction action on the second laser beam, but does not act on the first laser beam at all but is configured so as to allow the first laser beam to pass therethrough as the main beam of the 0-order beam.
Reference numeral 5 designates a parallel plate type semitransparent mirror, which is arranged in an oblique manner so as to reflect the first laser beam and the second laser beam having passed through the diffraction grating 4, and which allows return light, which will be described later, to pass therethrough and causes astigmatism. Reference numeral 6 designates a collimating lens, on which the first laser beam and the second laser beam reflected by the semitransparent mirror 5 are incident, and which is configured so as to change the incident light from divergent light to parallel light.
Reference numeral 7 is an objective lens, on which the first laser beam and the second laser beam that are parallel light having passed through the collimating lens 6 are incident, and which is configured so as to collect the first laser beam to a signal recording layer L1 of the first optical disc D1 and the second laser beam to a signal recording layer L2 of the second optical disc D2.
The objective lens 7 is configured so as to function as an objective lens having numerical aperture of 0.45 of CD standard or numerical aperture of 0.6 of DVD standard, by switching the diameter of an aperture stop provided so as to limit a light flux of each laser beam which is incident thereon according to the wavelength of the first laser beam and the wavelength of the second laser beam, or by being configured using a bifocal lens.
The first laser beam and the second laser beam, which is emitted from the two-wavelength laser diode 1 through an optical path constituted of the above-described optical components, is incident on the objective lens 7, and thus the first laser beam and the second laser beam are condensed to the signal recording layer L1 of the first optical disc D1 and the signal recording layer L2 of the second optical disc D2 by means of the light condensing operation of the objective lens 7, so that laser spots are formed which are suitable for reading signals recorded in the optical discs.
With such an operation, the laser spots are formed on the signal recording layer L1 of the first optical disc D1 and the signal recording layer L2 of the second optical disc D2, and at the same time, the laser beams are reflected from the signal recording layer L1 and the signal recording layer L2 as return light.
The return light of the first laser beam and the second laser beam reflected as such is incident on the semitransparent mirror 5 through the objective lens 7 and the collimating lens 6. The return light has been converted by a phase shift operation performed by the quarter-wave plate (not shown), as is well known, into linearly polarized light in a different direction, and thus it is not reflected by the semitransparent mirror 5 but passes through the semitransparent mirror 5.
The first laser beam and the second laser beam having passed through the semitransparent mirror 5 are given astigmatism, which is to be used in a focus control operation, and are applied to a photodetector 8. The photodetector 8 includes a four-divided sensor which is described in the above publication.
With the two-wavelength laser diode which is provided, in the same case, with the two laser elements configured to emit laser beams having different wavelengths, an optical path can be used for two laser beams, and thus there is such an advantage that optical composition of the optical pickup apparatus can be simplified.
Mainly used is the laser diode, included in the optical pickup apparatus configured to perform the operation of reading a signal recorded in the optical disc, which oscillates substantially at a single wavelength called a single mode, but it has such a problem that noise may be generated due to irregular reflection light reflected from the signal recording layer of the optical disc, which is called return light, when the laser beam is oscillated.
As a method for solving such a problem, used is a method for superimposing a high-frequency signal on a driving signal supplied to the laser diode, but there are not only a problem that such a method is expensive since a circuit for generating the high-frequency signal is required, but also a problem that measures should be taken against unnecessary radiation such as a shielding from an electromagnetic wave generated from superimposed integrated circuit components provided in order to generate the high-frequency signal.
As a laser diode that solves such problems, a laser diode capable of self-pulsation, that is, self-excited oscillation has been recently commercialized. FIG. 3 is a diagram for describing a shape of emission of the laser beam emitted from the laser diode, and the emitted laser beam is not in a circular shape but in an elliptical shape.
The laser beam emitted as such becomes an elliptical shape with a longer diameter in the perpendicular direction relative to a diameter in the parallel direction with respect to a junction portion, which is an active layer K making up the laser diode. A distance S between a virtual light emitting point P1 in the long diameter direction and a virtual light emitting point P2 in the short diameter direction of the laser beam is an astigmatic difference, and there is such characteristics that the magnitude of the astigmatic difference varies with types of the laser diode.
If the laser diode has the above-described astigmatic difference, astigmatism occurs which is caused by the astigmatic difference, and particularly the pulsation type laser diode has a large astigmatic difference, which leads to a problem that the astigmatism, caused by the astigmatic difference, becomes large.
Also, since the first laser element 2 and the second laser element 3 included in the two-wavelength laser diode are arranged in a manner slightly distant from each other, their optical axes are deviated. And there is such a problem that the astigmatism occurs with this deviation.